Field of the Invention
The invention lies in the field of semiconductor technology and relates to a method for fabricating a patterned laser containing metal-oxide.
Layers of this general type are provided for use, in future applications, inter alia as a capacitor dielectric in semiconductor memories, since these layers have high dielectric constants or are ferroelectric.
Therefore, the term metal-oxide-containing layer as used herein should be understood to be layers having a dielectric constant xcex5 greater than 10 and also ferroelectric layers. The latter are characterized in particular by their ability to maintain a remanent (permanent) polarization which can be reversed by an electric field. In the event of the polarization reversal, the polarization of the metal-oxide-containing layer in this case follows a hysteresis which is characteristic of the respective layer. To ensure that these layers have the dielectric or ferroelectric properties sought, they must generally be present in polycrystalline form.
Metal-oxide-containing layers comprise either a metal oxide, such as, for example, tantalum oxide (Ta2O3) or titanium oxide (TiO2), or a mixture of at least two metal oxides. The latter are also commonly designated as ABO class, where O denotes oxygen and A and B denote metals selected from the group of strontium, calcium, barium, bismuth, cadmium, lead, titanium, tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium and tallium. These metal oxides or metal oxide mixtures form crystal lattices or crystal superlattices, the latter being understood to mean the successive alternation of a plurality of sublattices. The general ABO class also subsumes substitution mixed crystals and isomorphs of the above metal oxides. A typical crystal structure is, for example, the layer perovskite structure which occurs for example in strontium bismuth tantalate (SrBi2Ta2O9).
A method for fabricating a patterned metal-oxide-containing and polycrystalline layer is described for example in U.S. Pat. No. 5,434,102. There, a metal-oxide-containing layer is initially applied to a substrate. The still amorphous layer is then briefly heated in order to induce crystallization nuclei and subsequently subjected to a thermal treatment. In the process, the metal-oxide-containing layer completely crystallizes into a polycrystalline layer which can then be patterned. It is unfavorable, however, that layers that are fabricated and patterned in this way exhibit stoichiometric deviations which, in particular in the case of very fine patterning on the micron and submicron scale (feature size approximately equal to or less than 1 xcexcm) can lead to impairment of the dielectric or ferroelectric properties sought. One consequence of these stoichiometric deviations is that the electrical loadability of the metal-oxide-containing layer is reduced. However, this is undesirable in particular in the case of large scale integrated semiconductor devices, for example in the case of semiconductor memories.
The object of the present invention is to provide a method of fabricating a metal-oxide-containing layer which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which leads to a durable layer.
With the above and other objects in view there is provided, in accordance with the invention, a method of fabricating a patterned metal-oxide-containing layer, which comprises the following method steps:
providing a substrate;
applying a substantially amorphous metal-oxide-containing layer to the substrate;
immediately following the applying step, performing a nucleation process to form crystallization nuclei in the substantially amorphous metal-oxide-containing layer;
patterning the substantially amorphous metal-oxide-containing layer; and
subsequently thermally treating the metal-oxide-containing layer to form a substantially polycrystalline metal-oxide-containing layer.
In accordance with an added feature of the invention, an electrode layer is deposited, before the patterning step, onto the substantially amorphous metal-oxide-containing layer and the electrode layer is patterned together with the metal-oxide-containing layer.
In accordance with an additional feature of the invention, the method comprises forming the metal-oxide-containing layer from a material selected from the group consisting of strontium bismuth tantalate, strontium bismuth niobate tantalate, lead zirconium titanate, barium strontium titanate, lead lanthanum titanate, lead lanthanum zirconium titanate, bismuth titanate, and a derivative of these metal oxides.
In accordance with a concomitant feature of the invention, the nucleation process is performed immediately following the applying step.
In other words, the method has as its basic concept that the thermal treatment during which the applied metal-oxide-containing layer, which is initially still essentially amorphous, is subjected to a crystallization process is carried out only after the patterning of the metal-oxide-containing layer. This produces particularly robust metal-oxide-containing layers which, for example, have a low leakage current. What is furthermore characteristic of the layers thus produced is that the stoichiometric ratios are only insignificantly affected by the patterning and the thermal treatment.
The layers thus fabricated prove to be particularly stable under electrical loading ranging to electrical breakdown. By contrast, in the case of the layers fabricated by previously known methods, macroscopically observable damage is often detected, which is attributable to changes in the metal-oxide-containing layer during patterning. In these layers, the damage is located predominately in the edge region and, therefore, has a pronounced effect in particular in the case of finely patterned layers (micron and submicron scale).
The relatively high mobility of some metal oxides is a suspected cause for explaining this damage. These metal oxides can diffuse relatively easily along grain boundaries or evaporate during the patterning of the thermally treated and hence polycrystalline metal-oxide-containing layer from the etched edge regions of the metal-oxide-containing layer. In particular the polycrystalline structure of the thermally treated metal-oxide-containing layer favors diffusion of highly mobile metal oxides which have a tendency to diffuse. Since these are often volatile as well and evaporate, in particular the edge regions of the metal-oxide-containing layer are disrupted. On account of the evaporation, there is a change in the stoichiometric ratio of the metal-oxide-containing layer and thus in the dielectric or ferroelectric properties sought. One consequence is e.g. a reduced breakdown strength.
Furthermore, the mobile metal oxides that have a tendency to diffuse can diffuse relatively rapidly as far as active regions of devices and influence the latter irreversibly.
By contrast, such damage is avoided by fabricating the metal-oxide-containing layer by the method according to the invention. The reason is that the metal-oxide-containing layer is patterned before its crystallization. In this case, the metal-oxide-containing layer is present essentially still in amorphous form, so that diffusion paths formed by grain boundaries are still absent. Rather, the amorphous metal-oxide-containing layer is only removed in layers by the patterning, with the result that possible evaporation of metal oxides can only take place from the topmost and thus extremely thin layer. Therefore, possible disturbances can only extend to a few atomic layers and, in contrast to already polycrystalline layers, due to the diffusion that is facilitated there, do not extend relatively far into the layer itself.
As a result, after the patterning, a largely undisturbed amorphous layer is thus present which is subsequently crystallized by the thermal treatment.
The metal-oxide-containing layer is essentially amorphous before the patterning and the thermal treatment. This state is present immediately after the deposition of the metal-oxide-containing layer and is characterized by a non-crystalline structure which may possibly already have a few crystallites, these being relatively small, however. Furthermore, essentially amorphous metal-oxide-containing layers do not yet have the dielectric or ferroelectric properties sought, i.e. the dielectric constant is relatively low, or as yet no remanent polarization or only a vanishing small remanent polarization can be brought about.
In order to improve or bring about the dielectric or ferroelectric properties, these layers must therefore be to subjected to crystallization, these layers then being essentially polycrystalline after crystallization. The aim here is crystallization that is as complete as possible, with the formation of relatively large crystal domains, since it is precisely these crystal domains which determine the desired properties.
The method according to the invention is preferably used for fabricating semiconductor memory elements in which the memory element is formed by a dielectric or ferroelectric capacitor. In this case, the metal-oxide-containing layer serves as a capacitor dielectric situated between two electrodes.
Preferably, before the patterning of the essentially amorphous metal-oxide-containing layer, an electrode layer is deposited onto the essentially amorphous metal-oxide-containing layer and is patterned together with the latter.
It has been shown that the subsequently deposited electrode layer protects the relatively sensitive metal-oxide-containing layer during patterning. Moreover, joint patterning of the electrode layer and of the metal-oxide-containing layer reduces the number of process steps.
Furthermore, it is expedient, before the thermal treatment, to carry out a nucleation process in order to form crystallization nuclei in the essentially amorphous metal-oxide-containing layer.
This step is, for example, a so-called Rapid Thermal Process (RTP), in which, by means of relatively brief heating of the metal-oxide-containing layer, first nuclei are formed in said layer but complete crystallization is as yet not effected. Complete crystallization is only effected afterward during the thermal treatment under optimum conditions therefor.
The nucleation process can either be carried out immediately after the application of the essentially amorphous metal-oxide-containing layer, or follow the patterning of the metal-oxide-containing layer.
The metal-oxide-containing layer preferably comprises strontium bismuth tantalate (SBT), strontium bismuth niobate tantalate (SBTN), lead zirconium titanate (PZT), barium strontium titanate (BST), lead lanthanum titanate (PLT), lead lanthanum zirconium titanate (PLZT), bismuth titanate (BTO) or a derivative of the above metal oxides.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating a patterned metal-oxide-containing layer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.